Approaches have been developed that allow the synthetic encoding of polypeptides and other biochemical polymers. An example of this approach is disclosed in U.S. Pat. No. 5,723,598, which pertains to the generation of a library of bifunctional molecules. One part of the bifunctional complex is the polypeptide and the other part is an identifier oligonucleotide comprising a sequence of nucleotides which encodes and identifies the amino acids that have participated in the formation of the polypeptide. Following the generation of the library of the bifunctional molecules, a partitioning with respect to affinity towards a target is conducted and the identifier oligonucleotide part of the bifunctional molecule is amplified by means of PCR. Eventually, the PCR amplicons are sequenced and decoded for identification of the polypeptides that have affinity towards the target. The library of bifunctional complexes is produced by a method commonly known as split-and-mix. The method implies that a linker molecule is divided into spatial separate compartments and reacted with a specific amino acid precursor at one terminus in each compartment and appended a nucleic acid tag which codes for this specific amino acid precursor at the other terminus by an orthogonal chemical reaction. Subsequently, the content of the various compartments are collected (mixed) and then again split into a number of compartments for a new round of alternating reaction with amino acid precursor and nucleotide tag. The split-and-mix method is continued until the desired length of polypeptide is reached.
This prior art method is constrained in its application because there must be compatible chemistries between the two alternating synthesis procedures for adding a chemical unit as compared to that for adding a nucleotide or oligonucleotide sequence. According to the prior art, the problem of synthesis compatibility is solved by the correct choice of compatible protecting groups as the alternating polymers are synthesised, and by the correct choice of methods for deprotection of one growing polymer selectively while the other growing polymer remains blocked.
Halpin and Harbury have in WO 00/23458 suggested another approach, wherein the molecules formed are not only identified but also directed by the nucleic acid tag. The approach is also based on the split-and-mix strategy to obtain combinatorial libraries using two or more synthetic steps. A plurality of nucleic acid templates are used, each having at one end a chemical reactive site and dispersed throughout the stand a plurality of codon regions, each of said codon regions in turn specifying different codons. The templates are separated by hybridisation of the codons to an immobilised probe and subsequently each of the strands is reacted at the chemical reaction sites with specific selected reagents. Subsequently, all the strands are pooled and subjected to a second partitioning based on a second codon region. The split-and-mix method is conducted an appropriate number of times to produce a library of typically between 103 and 106 different compounds. The method has the disadvantage that a large number of nucleic acid templates must be provided. In the event a final library of 106 different compounds is desired, a total of 106 nucleic acid templates must be synthesised. The synthesis is generally cumbersome and expensive because the nucleic acids templates must be of a considerable length to secure a sufficient hybridisation between the codon region and the probe.
In WO 02/074929 a method is disclosed for the synthesis of chemical compounds. The compounds are synthesised by initial contacting a transfer unit comprising an anti-codon and a reactive unit with a template having a reactive unit associated therewith under conditions allowing for hybridisation of the anti-codon to the template and subsequently reacting the reactive units. Also this method suffers from the disadvantage that a large number of nucleic acid templates initially must be provided.
The prior art methods using templates suffer from the disadvantage that encoding is dependent upon the recognition between the anti-codon and the template. The hybridisation between two oligonucleotides can occur in the event there is a sufficient complementarity between these. Occasionally, the hybridisation will occur even though a complete match between the oligonucleotides is not present. The effect is, in the event a plurality of transfer units are present then sometimes the codon sequence of the template does not correspond to the reactive unit actually reacted. This undesired effect is even more pronounced when the formation of library is intended because a plurality of templates and building blocks are supposed to find each other in the reaction media. When the hybridisation step is not completely correct, molecules will be generated that are encoded by the incorrect codons on the template. This will have two major effects on the selection process performed on the library. First, templates with a codon combination encoding for binding ligands will be lost in the selection process. Secondly, and may be more important, templates with a codon combination encoding for non-binding ligands will be enriched.
In an aspect of the present invention it is an object to provide a non-template dependent method for obtaining an encoded molecule, said method allowing for versatile chemistries to be applied in the formation of the encoded molecule, because the application of compatible orthogonal protection groups in the alternating formation of the encoded molecule and oligonucleotide tag can be avoided. The present invention in a preferred aspect intends to improve on the error prone hybridisation method previous suggested in the codon recognition process. Furthermore, it is an object of the invention to reduce non-specific reaction products formed. Thus, in an aspect of the present invention, the present method has an inherent proof-reading facility securing that the phenotype is accurately encoded by the genotype.